Perhaps it's a rival of Sakura's. In many fanfics, there are rivalries, whether it's for power, love, or recognition. If Sakura outmaneuvered this rival in an unfair way, like cheating in a competition for a coveted position or a magical power, the rival would hold a grudge and find it hard to forgive her.
There might also be a mentor - like figure in Sakura's life. If Sakura went against the mentor's teachings or used the knowledge she gained from the mentor for bad purposes, the mentor could be the main character who can't forgive her. For example, if the mentor taught Sakura about magic to protect the world, but Sakura used it to cause harm instead, that would create a huge rift.
It could be her friends from the present who somehow also get sent to the past with her. Maybe Tomoyo, who could be helpful in creating new outfits for Sakura to blend in with the past's fashion.
One of the main characters could be Sakura's mother. Mothers are often intuitive about their children's problems. She could be the one who eventually helps Sakura open up about her self - harm. Then, there might be a new acquaintance in Sakura's life, someone she meets randomly. This person could have had a similar experience in the past and be able to offer unique advice and support to Sakura.
Tomoyo would also be an important character. She would be very worried about Sakura. She might use her skills in making costumes to create some special baby clothes for Sakura. And she would be constantly by Sakura's side, taking care of her and trying to comfort her.
There could be Naruto. Since he's Sakura's friend, he would likely be involved, trying to help her deal with the curse mark. He might use his own powers and experiences to support her.
The Akatsuki members would be the main ones. It could include Itachi, who is known for his complex character and powerful abilities. His relationship with Sakura could be an interesting aspect, whether it's one of mutual respect or a more adversarial one.
A shy boy who also loves to read and he and Sakura bond over their shared love for books. They might form a study group together and support each other in their academic pursuits.
Sakura's friends like Naruto and Sasuke could be main characters. They might be the ones trying to find a way to save her. They could be using their skills and powers to look for a cure or a solution.
Maybe in the fanfic, Sakura has committed some unforgivable betrayal. For example, she might have broken a very important promise to the protagonist or she has caused great harm to someone the protagonist cares about deeply.
The main characters could be Sakura and the narrator. But there might also be a new love interest for the narrator. This new person could be a complete opposite of Sakura. For instance, Sakura could be very outgoing and the new love interest could be more introverted. This contrast might play a role in the narrator's change of feelings towards Sakura.
Definitely some of the Akatsuki members. Itachi could be one. He is a complex character with his own motives. His presence could add a lot of mystery to the story as Sakura might try to figure out his true intentions. Another could be Deidara. His explosive personality and love for destruction could create some intense confrontations with Sakura.